Environmental problems such as overflowing landfills are aggravated by the stability and longevity of some plastic materials. Recently, a great deal of attention has focused on developing plastics from water soluble biodegradable polymers such as polyvinyl alcohol (PVOH) PVOH is prepared by hydrolyzing polyvinyl esters, e.g. polyvinyl acetate Depending on the amount of hydrolysis, PVOH will have varying degrees of water solubility. The environmental appeal of PVOH is that the resulting solute is biodegradable.
The utility of polyvinyl alcohol has been severely limited by the fact that it decomposes before melting. For example, melt extrusion processing of plastics requires formation of a chemically stable "melt" of the polymer that resolidifies upon cooling without losing its desirable characteristics. Until recently, means for melt extruding materials possessing the desirable characteristics of polyvinyl alcohol have not been available.
Thermoplastic materials offering the desirable characteristics of polyvinyl alcohol are described in U.S. Pat. Nos. 4,675,360 and 4,618,648. Those references describe copolymer compositions of PVOH and poly(alkyleneoxy)acrylate that are reportedly extrudable into solid films. The PVOH copolymer compositions are thermoplastic yet retain PVOH's water solubility, biodegradability and strength characteristics.
Similarly, U.S. Pat. Nos. 4,600,746 and 4,575,532 describe polyvinyl alcohol alloys that have good separation of melting and decomposition points and are useful as gas barrier materials. Among other things, the materials can be extruded into gas barrier films, sheets, tubings, coatings, bottles and profiles The films have low gas permeability and water absorptivity characteristics.
Notwithstanding these advances, the foregoing references do not describe materials or methods for processing polyvinyl alcohol materials into foams. By foam is meant a solid plastic material of expanded cellular construction.
Foams of PVOH have been prepared, however. U.S. Pat. No. 3,695,989 describes cold water soluble foamed films fabricated from PVOH, polyvinyl pyrrolidone, and mixtures thereof. Those formulations are doctor knife casted aqueous mixtures containing foaming agents. The utility of those films is limited because they are susceptible to all forms of moisture and must be protected from rain, dew and high humidity atmosphere.
Similarly, U.S. Pat. No. 4,692,473 describes a water borne foamable composition that may incorporate minute quantities of polyvinyl alcohol. The polymer formulations of that description are always water borne and are not suitable for melt extrusion.
The foregoing references do not describe compositions of polyvinyl alcohol materials that can be extruded into foams. Nor do they describe methods whereby known compositions of polyvinyl alcohol materials can be extruded into foams. More particularly, the foregoing references do not describe a compatible blowing agent for such compositions.
A composition for extruding a foam will produce a solid plastic material having an expanded cellular construction when fabricated by melt extrusion processes. The blowing agent is a relatively volatile component mixed into the melt. When the melt is extruded, the blowing agent must volatilize out of the melt before solidification. The volatilization of the blowing agent forms gas bubbles, which give the foam its expanded cellular construction.